In the manufacture of gaskets, for example automobile carburetor gaskets, it is known to associate the compressible gasketing material with a rigid non-compressible component which provides a stop or positive limit to the compression of the gasketing component. The gasketing component is usually a relatively thick flat sheet, ported for the flow of liquids and/or gases. The rigid compression limiting stop may be in the form of a ferrule, bushing, or grommet, secured in an opening in the gasketing material. The rigid stop may also provide the bolt hole. As the bolts are tightened in mounting the gasket (for example, to form a seal between an engine intake manifold and the carburetor throttle body) the gasket component is compressed to form the seal, but the compression is limited by the point at which the opposed engine components bottom out on opposite faces of the rapid stop. Such gaskets are shown in Farnham U.S. Pat. No. 3,655,210.
In a reverse arrangement, the rigid compression limiting component may form an outer boundary or rim around the periphery of the gasket component, the latter being seated in the former, as shown in Olson U.S. Pat. No. 3,355,181; Moyers U.S. Pat. No. 3,195,906; and Papenguth U.S. Pat. No. 3,215,442.
In the manufacture of both such types of two-component gaskets, it is desirable to mold the rigid component in situ against the preformed compressible gasketing component. The rigid component may be molded from a flowable resin which may be a thermosettable plastic. It is desirably injection molded in, or around, the compressible component. The compressible component must of course be thicker than the rigid, compression-limiting component, so as to be compressed in use and thereby form a seal.
In the manufacture of such gaskets, the compressible material is conveniently preformed, for example by die-cutting it to size. The preformed part is then seated within the cavity of an injection mold, and the molding material is injected into a cavity one edge of which is formed by the side of the compressible material. As the molding material cures or sets up, it also bonds integrally to the side of the compressible component. This seals the side of the gasket to inhibit wickage and fluid transfer laterally.
In making gaskets by the process described, it is difficult to get a reproducible, durable line of demarcation between the two components, that is, where the rigid component meets the compressible component. In particular, the molding material under the pressure of injection, tends to form an irregular flashing which projects onto the gasketing face of the adjacent compressible material. The flashing appears even though the compressible material is thicker than the rigid component. This flashing, since it is not elastic or compressible, can impair the seal, and in any event is undesirable from an appearance standpoint.
I have found that this flashing occurs as the result of either or both of two factors; permanent edge deformation of the compressible component when it is precut, and secondly, elastic compression of that component between the mold halves as the second component is injection molded to it. As a result of the die-cutting step, the edge of the compressible material, where the gasketing face meets the side which is to be bonded to the plastic molding, may become somewhat deformed and rounded as a sloping shoulder surface. The side of the compressible material does not meet the face at a precise angle. This rounded edge is presented to the flowable resin, in the molding, which flows onto the sloping shoulder as an irregular and unsightly flashing. But whether or not a rounded shoulder is present, the molding pressure will escape from the cavity under the high pressure of molding, and will flow over the face of the gasket as a flashing.
It has been the objective of this invention to provide a method and structure whereby this flashing can be prevented from forming in molding, with the resultant production of a crisp, sharp join line between the bonded-together rigid and compressible components, even though the latter presents a rounded corner in standing proud of the former.
More particularly, it has been found that this flashing can be altogether prevented by providing an elongated, narrow compression rib on at least one of the injection mold halves, the rib being positioned adjacent and generally parallel to the edge of the compressible component so that it encircles the latter. In molding, the resin flows up to and against this rib, which forms an edge of the cavity in which the molding material is injected. The ridge has sufficient depth, relative to the surface of the mold, that it bears upon the compressible component and, like a dam, prevents the flow of molding material past it. The rib may overlie or straddle the edge of the face of the compressible material, but in any event its cross section must at least partially overlie the compressible component. In the resultant product, the impression of this rib is manifested as a permanently deformed channel, on one side of which forms the edge of the rigid component. The edge is sharp and reproducible, and the formation of flashing is obviated.